Many types of disposable medical equipment (such as, for example, arterial filters for cardiopulmonary surgery) consist of rigid components (e.g. filter housings) which are assembled at the factory with flexible tubing, sterilized, and then marketed in assembled form in sterile packages which frequently remain on hospital shelves for a year or longer.
For a number of reasons including clarity, strength, flexibility, ease of application, bonding ability, and cost, the preferred material for the rigid parts of such assemblies is polycarbonate, while the preferred material for the flexible tubing parts is plasticized polyvinyl chloride (PVC).
Unfortunately, it has been discovered that stress cracks, ranging from minor surface crazing to extensive cracks deep enough to cause catastrophic failure of the part in use, tend to develop on the polycarbonate parts, sometimes during sterilization but more often during shipping and storage. It has now been found that the source of the problem is the slow migration along the surface of the polycarbonate parts of the phthalate plasticizers commonly used in the manufacture of commercial medical-grade PVC tubing. Contact of the plasticizer with areas that have been subjected to high molding stresses weakens the polycarbonate part in those areas and allows stress cracks to form. The problem is exacerbated by the alcohol or solvent lubricants and bonding agents which are typically used to facilitate assembly of the parts.